Electroheating is a method of rapidly heating substances, such as solid or liquid foodstuffs, by passing a current through the material, wherein the material acts as a resistive heater. Such rapid heating methods are disclosed in applicant/assignee's U.S. Pat. Nos. 4,739,140; 5,583,960; 5,636,317 and 5,863,580, the disclosures of which are incorporated herein by reference.
The fluid to be electroheated must be in contact with a large area of the electrode in order to prevent a high current density on the electrode that might lead to arcing. U.S. Pat. Nos. 5,583,960; 5,636,317 and 5,863,580 describe apparatus for increasing the electrode contacting area and thereby reducing the current density. The apparatus includes a narrow tube which terminates at both ends thereof in funnel-like cones. The electrode is the size of the large base of each cone.
A problem exists when attempting to electroheat semi-solid materials, such as coagulated proteins or dough. It is difficult to form good electrical contact between a flat electrode and the semi-solid material. The narrow tube apparatus of the abovementioned patents solves this problem by providing good contact area and low current density at the cone ends. However, although this arrangement provides low current density, it increases the dwell time in the electroheater, since the volume of the cones is much larger than that of the narrow tube. The increased dwell time presents another problem by making it difficult if not impossible to pass the semi-solid material through the electrode, since the semi-solid material tends to thicken and harden during the dwell time.
Another problem associated with electroheating of a biological fluid, is that the fluid contacts the electrode. The electrodes are usually made of graphite, which is preferable to metal because metal ions can dissolve in the contacting fluid, whereas graphite does not. Nevertheless, even with graphite electrodes, there is an electrolytic reaction with the fluid, and the fluid becomes reduced. Although in some cases this can be beneficial, such as in recovery of oxidized vitamin C in electroheated orange juice, nevertheless in many cases this is not desirable.